The present invention relates generally to induction devices such as transformers which are encapsulated in a polymeric medium. More specifically, the invention relates to dry type induction devices which are enclosed within an encapsulant.
The current state of the art of the encapsulation of dry type induction devices such as transformers involves the use of solvent-containing and also solventless encapsulants or impregnants. The silicone resins have been used for such encapsulation and may involve the use of solvent. Thermoset polybutadienes have been used in place of the silicone resin and have been used without solvent. Generally, the polybutadienes have lower cost than the silicones and also, in a general way, the thermoset polybutadienes have been formulated and employed as direct replacement for the more costly and problematic silicone compositions.
Also, in a general sense, the use of the silicone has not been displaced by the solventless thermoset polybutadienes in one significant segment of this technology. The silicone compositions have continued to be used in those applications where access to the induction device is desired for some reason after the encapsulating polymer has been cured. In the case of hard cure thermoset polybutadienes, the efforts to liberate the induction device such as a transformer from the encapsulant has lead to the destruction of the contained apparatus because of the hardening of the encapsulant. Accordingly, the solventless thermoset polybutadiene composition cannot be employed in encapsulating apparatus which may, after cure has been effected, be subjected to some modification of the apparatus.
The more expensive silicone compositions have been employed on a continuing basis for those installations which may be subjected to or have a requirement for modification or adjustment after the encapsulation and curing has been completed. For transformers, this includes post cure adjustment, non-destructive tear-down or other rework which may be carried out, or which one may desire to carry out, after the encapsulation has been completed. Where encapsulation with the relatively hard curing thermoset polybutadiene has been carried out, then such non-destructive tear-down or rework is not feasible because of danger of damage to the equipment as well as the great difficulty of trying to remove the hard cure thermoset polybutadiene without doing damage to the equipment.
A practical basis for the use of the disassembly technique in repair or modification of transformers involving the use of the silicone resins as encapsulants is the repair of the transformer after removal from the enclosed container and the removal of the coil from the core of the transformer by dismantling the core. The coil may then be repaired or may be completely replaced with a new coil but the magnet steel of the transformer is saved and reused. The coil, if it is repairable, may be repaired and returned to the transformer in good condition so that it can pass the normal test requirements in a final inspection. By contrast, if the transformer cannot be removed, then the transformer, including the core and the coil and magnet steel of the core as well as the basic housing, must be written off as a loss.
Pursuant to one aspect of the present invention, an elastomeric polybutadiene may be employed in forming a novel elastomeric encapsulant formulation and the novel formulation may be cured to a dry state and when so cured be of a consistency such that the core can be dismantled from the transformer. The coil can be removed from the core to be repaired or to be replaced. The transformer can be repaired or reconstructed with the same magnet steel thus saving the cost of the magnetic steel and depending on the condition of the core, saving also the cost of the coil. As a consequence, the core and coil does not have to be scrapped and written off as a loss.
Another application for encapsulants of magnetic devices is the encapsulation of devices which use strain sensitive torroidal ferrite cores. It has been a practice in the industry to coat such cores by a very high cost process known as the "parylene process", which is a high technology process available under license agreement with Union Carbide Company. The parylene process involves the deposition of a strain-free, continuous polymer film based on pyrolysis of a composition followed by subsequent condensation. Because of the nature of the process, providing the necessary equipment and paying the necessary cost, makes the process an expensive one. Surprisingly, it has been found that it is feasible to provide an elastomeric formulation based on elastomeric polybutadiene which serves the purpose of encapsulating the strain sensitive torroidal ferrite cores and it is feasible to do so at a substantially lower cost. It has been found in fact that through use of the process and composition of the present invention, encapsulated strain sensitive torroidal ferrite cores can be suitably encapsulated to provide three significant advantages as follows.
1. The novel elastomeric composition has a low, unvarying dielectric constant,
2. The novel elastomeric composition further has a low dissipation factor, and
3. It has been found that the novel elastomeric composition is virtually strain-free.
Strain sensitive toroidal ferrite cores are used to make high frequency transformers or high frequency conductors to be used in AC and DC power supplies. The prior art technology for coating the cores involved an expensive coating produced by the parylene process as described above or painting or, as an alternative, a polyester coating. However, these prior art coatings, except for parylene, are very hard. During the aging of the magnetic component, specifically, the strain sensitive torroidal ferrite core, the prior art hard coatings become harder and harder as the coating material is aged due to oxidation and heat. This hardening puts a stress on the ferrite core and changes the magnetic properties of the core. For example, using the prior art processes, a high frequency inductor has been aged and changes from one value of inductance to a lower value of inductance because of the strain that prior art hard materials put on the core during the aging period.
Further, it has been observed that the prior art encapsulant materials are subject to change of their electrical characteristics during the aging process. For example, the electrical characteristics, specifically, dielectric constant and dissipation factors, have been observed to change during the aging process. It is further known that in a high frequency inductor or transformer, a change of the dielectric constant or dissipation factor of the encapsulant changes the characteristics of the magnet component. As a result of such change, it has been observed that the electrical characteristics of the magnet component will stray outside of the specified limits for either the inductance or capacitance or undergo other losses of electrical characteristics due to aging.
The patent to McElroy, U.S. Pat. No. 3,678,121, describes the preparation of a high vinyl liquid polybutadiene, but does not describe any scheme by which encapsulation such as is taught in the present invention may be accomplished.
The Bockstie U.S. Pat. No. 4,147,477 concerns the use of high vinyl butadiene polymer for coating electrical components. The components which Bockstie refers to are resistors and the composition which he describes would lend itself more to a hard curing thermosetting encapsulant than it would to an elastomeric type of encapsulant.
The U.S. Pat. No. 3,079,295 teaches resinification by grafting of styrene or vinyl toluene but does not teach the formation of an elastomeric encapsulant formulations.
Another feature of the present invention, in another of its aspects, is the provision of encapsulant compositions which are usable at higher temperatures of 220.degree. C. or the like.
Some prior art patents relate to the use of polybutadiene as encapsulant composition in a general way. One such patent is Mees et al U.S. Pat. No. 3,970,723 which is assigned to the same assignee as the subject application. It does describe use of a polybutadiene for encapsulation of transformers. The polubutadiene composition taught is not taught to be useable at elevated temperatures nor is there any technique, method or means taught by which it may be useable at such higher temperatures. Because the composition of the Mees et al U.S. Pat. No. 3,970,723 is a hard curing thermosetting polybutadiene, the material does cure to a hard state and the hard cure of the encapsulant effectively prevents the modification or repair of the magnetic element which is encapsulated therein.
As indicated above, another deficiency of prior art encapsulants for induction devices such as transformers is the limitation on the temperatures at which they can operate either for short time intervals or for extended time intervals unless very high cost materials such as silicones are used in the encapsulation.
According to the known prior art, one high temperature, e.g. 220.degree. C., insulation system for encapsulation of induction devices such as transformers consists of a polyimide magnet wire insulation and, specifically, the Dupont ML polyimide magnet wire insulation together with a silicone varnished treatment or a high temperature polyester treatment. Such combinations are capable of operating at 220.degree. C. Another combination of insulations which have been used in connection with transformers for high temperature operation is the combination consisting of a Dupont NOMEX wrapped wire insulation along with a silicone varnished treatment of the transformer. However, it is also known that the Dupont ML wire insulation, as well as the NOMEX insulation, are expensive and also it is known that the silicone varnish is very expensive.
There are, in fact, two sets of deficiencies of the presently used prior art encapsulant systems. A first of the deficiencies as described above relates to the failure of the lower cost systems involving polybutadiene to permit access to the encapsulated apparatus or equipment at a later time following curing. A second deficiency is that there has been no low cost encapsulant systems which permit operation at desired elevated temperatures such as those described immediately above.
However, pursuant to the present invention, there is provided a system for encapsulation of inductive devices which can be employed either in the encapsulation of such devices in a fashion which permits later liberation of such devices without their destruction and the system of the present invention does also permit the option of a relatively high temperature operation without loss of important operating properties of the system components or of the apparatus.
Further, the present invention provides a system which permits both operation at elevated temperatures and liberation of the encapsulated induction apparatus.